1. Field of Invention
The present invention relates to improvements in devices for measuring the presence or absence of flow of a gas above a predetermined threshold for flow in one direction with a differing threshold for flow in the opposite direction, the flow of such gas being confined between an inlet and outlet ports.
2. Description of the Prior Art
Customarily, gas flow in a pipe is measured as a pressure difference between an upstream location and a downstream location, a section of the pipe at an intermediate location containing either a resistance element to insure laminar gas flow therethrough or a defined discontinuity to insure a predetermined turbulent gas flow. Other useful methods involve the direct measurement of mass flow employing such devices as rotation vanes in a fixed location or spheres lifted in a tapered pipe. Still further methods involve the measurement of transit time of an acoustic signal along the direction of gas flow.
All of these methods have certain common disadvantages such as the complexity of apparatus required to perform the methods and their inability to combine sensitivity for low gas flows with low flow resistance, as well as their sensitivity to contamination by dirt or moisture. These disadvantages are overcome in the apparatus of the present invention.